The present invention relates to a method for diagnosing a quality signal which is provided by a quality sensor which is used in a reagent metering system, wherein the reagent metering system meters a urea/water solution into the exhaust gas duct of an internal combustion engine upstream of an SCR catalytic converter.
The invention further relates to a specially configured control device for carrying out the diagnosis method according to the invention.
Furthermore, the present invention relates to a control device program which executes all of the steps of the diagnosis method according to the invention when said control device program runs on a control device.
Finally, the present invention relates to a control device program product with program code, which is stored on a machine-readable carrier, for carrying out the diagnosis method according to the invention when the program is executed on a control device.
In order to adhere to the ever more stringent exhaust gas legislation, it is necessary to reduce nitrogen oxides in the exhaust gas of internal combustion engines, in particular of diesel engines. For this purpose, it is known for an SCR (Selective Catalytic Reduction) catalytic converter to be arranged in the exhaust gas duct of internal combustion engines, the said SCR catalytic converter reducing the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine, in the presence of a reducing agent, to form nitrogen. In this way, the fraction of nitrogen oxide in the exhaust gas can be considerably reduced. Ammonia (NH3) which is admixed to the exhaust gas is suitable for the reaction to take place. Therefore, NH3 or reagents which split to form NH3 are used as reducing agents. In general, for this purpose, use is made of an aqueous urea/water solution as a precursor of the reducing agent, which aqueous urea/water solution is injected into the exhaust gas duct upstream of the SCR catalytic converter. Ammonia, which acts as reducing agent, is formed from the said solution. A 32.5% aqueous urea solution is commercially available under the trade name AdBlue®.
The urea/water solution can be refilled by an operator of the internal combustion engine. A check to determine whether the refilled substance is actually the correct urea/water solution or whether the urea concentration in the urea/water solution corresponds to the relevant standard ISO 22241 or DIN 70070 ensures that the required NOx reduction rates are complied with. The legislature requires a check in order to be able to identify possible attempted fraud when refilling the urea/water solution and to initiate corresponding measures, such as limiting the number of possible internal combustion engine starting operations or reducing the internal combustion engine torque for example.
A quality sensor which detects the quality of the urea/water solution is provided for diagnosing the urea/water solution. A reagent metering system comprising a quality sensor of this kind is known, for example, from laid-open specification DE 101 39 142 A1.
Laid-open specification DE 10 2015 212 622 A1 (not previously published) describes a method and an apparatus for connecting a quality signal which is provided by an ultrasound quality sensor which is used in a reagent metering system. The ultrasound quality sensor ascertains the sound propagation time in a urea/water solution which is stored in a tank as a precursor of the reagent ammonia. The quality signal which is provided by the quality sensor is based on a time-based measurement.
The so-called SENT protocol, which has been specified by the “Society of Automotive Engineers” (SAE) under the name J2716 and can be viewed at www.SAE.org, was developed specially for transmitting sensor data, in particular in motor vehicle applications. The SENT protocol is also described with reference to analysis with an oscilloscope in the specialist journal “Elektronik Praxis”, Vogel-Verlag, May 16, 2013, also published at http://www.elektronikpraxis.vogel.de/messen-und-testen/articles/405111.
The invention is based on the object of specifying a method and an apparatus for diagnosing a quality signal which is obtained on the basis of a time-based measurement, which is intended to ensure proper functioning of the quality sensor.